However, the website claims that it provides a variable pot of R L which is not true as you can clearly see in the circuit diagram, the pot is used to set the variable voltage for op-amp and has nothing to do with R L. In my sainsmart MQ-137 gas sensor the value of RL was 1K and was located here as shown in the picture below. Grauonline has already done this work for us and the circuit diagram of the MQ gas sensor board is given below.Īs you can see the resistor R L (R2) is connected between the Aout pin and the ground, so if you are having a module the value of R L can be measured by using a multimeter in resistance mode across Vout pin and Vcc pin of the module. If you have already purchased a module, then you should track your PCB traces to find the value of your R L in the board. So make a note of which value you are using, a value of 47k is recommended.

The resistor R L plays a very important role in making the sensor work. Connect one set to supply voltage and the other to your analog pin. The pinout and connection of the sensor is shown belowĪs you can see you just have to connect one end of ‘H’ to supply and the other end of ‘H’ to ground. For now, let’s assume you have purchased the sensor.

So if you have purchased the module already then you have to perform a small hack which will be discussed further. If your purpose is to measure only ppm then it’s best to buy the sensor alone since the module is good for only using the Digital pin. The MQ gas sensors can either purchased as a module or just as a sensor alone. I am explaining things from the bottom without any libraries so that you can use this article for any Gas sensor available with you. So after fiddling around online for a long time I finally found how to use these MQ series gas sensors to measure ppm using Arduino. Most tutorials either deal with only the Analog values or introduce some constants which are not reliable for measuring all type of gas. With the sensor in hand I went through all the available tutorials and found that there has no proper documentation on how to measure the gas in ppm. Here I will be using the MQ-137 sensor from sainsmart to measure ammonia in ppm.

In order to use the correlation function, you now need to compute the Ro value.We have already used MQ2 for smoke sensing and MQ-135 for Air quality monitoring project. Those values have been found using the method proposed in the above post, and the figures posted below. Someone ask me about sensor and correlation coefficients of various gasses, so here I will post some correlation coefficient for the a few MQ sensor and gasses. In the post linked above I've discuss a method to correlate the sensor resistance to a gas ppm using the datasheet curve named ' sensitivity characteristics of the MQ-135' and the formula:

How to calculate ppm from mq4